Bone marrow transplatation (BMT) is now an important therapeutic alternative for many patients with leukemia, as well as in a number of non-maligant conditions. The usefulness of BMT is limited, however, by the dangers of graft-versus-host disease and by the vulnerability of recently transplanted, immunodeficient patients to infection. We propose to use newly developed limiting dilution methods to clarify the cellular basis for post-transplant immunodeficiency. The limiting dilution technique permits us to measure the number of functional T cell precursors for helper, cytotoxic, and proliferative functions. We can also measure the ammount of functional effect (e.g. IL-2, or number of killer cells) produced by individual T cell clonal precursors in BMT recipients. We will determine the pace, and the eventual extent, of helper and killer T cell repopulation after transplatation in various clinical subgroups of the BMT population. We will compare the results of these frequency tests to data obtained by conventional in vitro assays of T cell regeneration, and by analysis of surface marker antigens, to learn under what circumstances immune dysfunction is associated with an underlying deficit in precursor frequency. We will look for correlations between precursor T cell regeneration and potentially relevant clinical variables: diagnosis, degree of genetic disparity, donor and recipient age, and others. Our preliminary results suggest that the T cells that repopulate most BMT recipients are in fact functionally impaired; we paln to begin an analysis of the basis of this defect by examining antigenic and metabolic indices of early T cell activation in BMT patients. Finally, we will use LDA assays to evaluate new methods for depleting mature T lymphocytes from allogeneic marrow prior to transplant in order to prevent GvHD.